The subject invention relates to a central tire inflation (CTI) system that utilizes passages formed within rotating wheel shafts to provide an easily sealed flow path to conduct air from a vehicle air supply to a tire assembly on a drive axle. The passage in each wheel shaft includes a fluid inlet formed at an inboard shaft end that communicates with the air supply and a fluid outlet formed at an outboard shaft end that communicates with the tire assembly.
CTI systems automatically supply air to vehicle tires when certain predetermined conditions are met. In a typical situation, the CTI system will automatically add air to a tire that becomes under-inflated during vehicle operation. CTI systems typically include sensors and pressure regulators for monitoring the pressure in each of the vehicle's tires. When pressure in any tire drops below a predetermined level, the system automatically supplies air to that tire until a desired tire pressure is achieved.
CTI systems include an air tank that is mounted on the vehicle. Air is supplied from the tank to the tires by using various methods. CTI systems can be incorporated into drive or non-drive axles. Traditionally, for either type of axle, CTI systems include an air connection that allows air to be supplied from a non-rotating air source to rotating tires. Some systems include an air connection that is mounted for rotation with a wheel hub assembly at one end, and which is mounted at an opposite end to a non-rotating axle tube for a non-drive axle. In these systems, air is supplied from the tank to an interior of the axle tube. Air from inside the axle passes through the air connection and is conducted to the rotating tires.
Traditionally, CTI systems for drive axles have been more difficult to incorporate into conventional wheel ends. Typically, these CTI systems use drilled passageways in rotating wheel hubs in combination with drilled passageways in non-rotating spindles. This configuration requires large, expensive seals, is difficult to assemble, and is expensive to machine. Further, wheel hubs for drive axles using disc brakes and single tires often do not have enough packaging space to accommodate drilled passageways. Thus, using a CTI system for such a configuration is not practical.
Another disadvantage with CTI systems that utilize passageways formed within the wheel hub, is interference with wheel bearing operation. Typically, the wheel hub is supported for rotation relative to the spindle on a pair of bearings. These bearings must be properly lubricated in order to operate efficiently and effectively. Forming an air passageway through the hub can be detrimental to bearing operation, especially if one of the seals fails or leaks. Further, the industry is moving toward the use of unitized bearings, which require a permanently sealed unit. Conveying air through the bearing unit is contrary to this concept.
Other CTI systems for drive axles have utilized a pair of drilled passageways formed within a rotating axle shaft. A radial air passage is drilled into the axle shaft to connect with a laterally extending passage. An air coupling surrounds the axle shaft at the radial air passage location to provide a sealed circumferential air compartment between an external surface of the rotating axle shaft and an interior surface of the air coupling. Air is transferred from the air source and into the radial passageway by way of the air compartment.
This system has several disadvantages. The air coupling is a complex mechanism requiring several component pieces and multiple seals, which makes the coupling very expensive. Further, assembling the coupling onto the axle shaft is difficult and time consuming. Also, the system is difficult to maintain and repair.
A more cost effective CTI system for a drive axle is needed. The system must be easily incorporated into the drive axle without requiring extensive machining, assembly, or requiring passageways formed within the rotating wheel hub. Further, the system should be easy to maintain and repair as well as overcoming the other above-mentioned deficiencies with prior art systems.